1. Field of the Invention
The present invention relates to an electrostatic printing apparatus such as a xerography-basis printer or a xerography-basis copying machine, and more particularly to a fixing system that is adequately used for printing images on both sides of a paper or a plastic sheet, and an electrostatic both-side printing apparatus which is capable of simultaneously printing images on both sides of an image printing medium, such as a paper or a plastic sheet, by one process.
2. Description of the Related Art
The both-side printing system for printing images on both sides of an image printing medium, e.g., a paper, is categorized into two systems; a first printing system of the one-process type in which toner images transferred onto both sides of an image printing medium are simultaneously fixed thereto, and a second printing system of the two-process type in which a first toner image is transferred and fixed on one side of an image printing medium, and then a second toner image is transferred and fixed onto the other side of the image printing medium.
The concept of the both-side printing system of the one process type has been known for long. Japanese Patent Examined Publication No. Sho. 51-13022, for example, discloses the construction of this type of the both-side printing system, and further a technique to correct an out-of register between the images transferred onto both sides of the image printing medium by using an exposure timing and the shape of a photoreceptor.
Most of the both-side printing systems, available on the market, are of the two process type. The both-side printing system of the one process type has never been put into practical use.
There are some reasons why the both-side printing system of the one process has never been put into practical use. The reasons are as follows:
(1) Unstable transferring of toner images to both sides of a image printing medium: PA1 a) The both-side printing system disclosed in Japanese Patent Unexamined Publication No. Sho. 51-13022 as referred to above uses transferring corotrons for transferring toner images from the first and the second toner image bearing means to an image printing medium. PA1 b) When the transferring of the second toner image is carried out, the first toner image 107 is not yet fixed to the image printing medium 110. In this state, an electric field developed by a transferring corotron 120 causes the toner of the first toner image to move to the corotron wire, although it depends on the polarity of the second toner image. As a result, the first toner image may be disturbed or the inside of the corotron may be soiled with the toner. PA1 (2) When the image printing medium 110 is transported at a high speed, part of the toner of a toner image attached but not fixed to the lower side (reverse side) of the image printing medium 110 drops by gravity. This leads to the deterioration of the image quality. PA1 1) The problems of the heat roll fixing system, which arises when it is used for fixing toner images on both sides of the image printing medium, will be described with reference to FIGS. 2A and 2B. PA1 2) In the electrostatic image printing apparatus using such a fixing system, problems of character blur and offset arise when the upper and the lower fixing rolls are retracted from the image printing medium at the time of starting and stopping the both-side printing operation, and the fixing rolls are compressed together.
The problems which arise in the process of transferring the toner images to both sides of the medium in the publication will be described with reference to FIG. 1.
As shown, a first toner image bearing means and a second toner image bearing means are disposed on both sides of an image printing medium 110. The first toner image bearing means consists of a photoreceptor in the form of a drum, which is constructed such that a photoconductive layer 102a made of SeTe is layered on the surface of a conductive substrate 101a made of aluminum. Similarly, the second toner image bearing means consists of a photoreceptor in the form of a drum, which is constructed such that a photoconductive layer 102b made of SeTe is layered on the surface of a conductive substrate 1b made of aluminum.
A charger 103a applies positive charge onto the surface of the first toner image bearing means, and an exposure unit 105a irradiates the same with light containing image information, to thereby form an electrostatic latent image thereon. A developing unit 106 develops the latent image into a visual image 107 with toner. Reference numeral 112a designates an erasing lamp, and 113a, a drum cleaner.
In transferring the first toner image from the first toner image bearing means to the image printing medium 110, a transferring corotron 131 applies negative charge 130 to the lower side of the image printing medium 110. Accordingly, an electrostatic force exerts between the positively charged second toner image and the negative charge 130, so that toner of a second toner image, positively charged, formed on the second toner image bearing means, flies to the image printing medium 110 before an image transferring position. This toner flying phenomenon is called a "before-transfer toner scattering". The phenomenon greatly deteriorates the quality of the resultant image.
The above-mentioned Japanese Patent Unexamined Publication No. Sho. 51-13022 also discloses a radiated heat fixing system of the noncontact fixing method for the fixing system of simultaneously fixing toner images formed on both sides of the image printing medium. The contact fixing system using the fixing rolls, which is capable of simultaneously fixing the toner images on both sides of the image printing medium has not been developed and marketed. To fix toner images on both sides of the image printing medium by the contact fixing system, a toner image is fixed on the obverse side, for example, of a paper, the paper is reversed, a toner image is formed on the reverse side of the paper, and transported to a fixing stage where it is fixed on the reverse side. The pressure roll of the fixing rolls contains a heat source to supply the quantity of heat to the paper for making up for the deficiency of heat.
The heat roll fixing system has the following problems.
The heat roll fixing performance is determined by heat roll temperature, nip pressure, and nip width (fixing time). To secure the nip area, one of the fixing rollers is a pressure roller with an elastic layer formed thereon. As shown in FIGS. 2A and 2B, the upper fixing roller 15 is a hard roller as a drive roller, and the lower fixing roller 16 is a soft roller with an elastic layer thereon as a follower roller.
The fixing roller 15 is constructed such that a release resin layer 20a made of fluorine resin, for example, is formed on the surface of a metallic roll 21a made of aluminum, for example, and the metallic roll 21a contains a halogen lamp 22a. The fixing roller 16 is constructed such that an elastic layer 23 (of h thickness) made of silicon rubber is formed on the surface of a metallic roll 21b made of aluminum, a release layer 20b made of fluorine resin is formed on the elastic layer, and the metallic roll 21b contains a halogen lamp 22b therein. Both the fixing rollers are compressed together by a fixed load, to thereby form a nip area of the width W. Normally, the radius r.sub.1 of the upper hard roll 15 is equal to the radius r.sub.2 of the lower soft roll 16. The hard roll is used as a drive roll, and the soft roll is used as a follower roll. When those fixing rolls are compressed together, the lower soft roll is deformed since it has the elastic layer 23. The effective radius r.sub.2 is changed to the radius r.sub.2, in the nip area. As a result, the effective radius r.sub.2, of the lower soft roll &gt;the radius r.sub.1 of the upper hard roll. When the rotation angular speeds of the upper and the lower rollers are equal to each other, a moving speed v2 of the upper hard roll &gt;a moving speed v1 of the lower soft roll, causing a speed difference. Accordingly, as illustrated in FIG. 2A, a moving speed of the lower side of the image printing medium is larger than that of the upper side thereof, so that stresses Fc and Fe of different directions act on the toner images on the upper and the lower sides of the image printing medium. Therefore, the fixed toner images 19a and 19b after passing through the nip are blurred, viz., a character blur occurs (FIG. 2B). This character blur phenomenon is not observed when an image is printed on one side of the image printing medium, and when an image is printed on one side of the image printing medium, and another image is printed on the other side thereof.